Double-Sided Metasurfaces for Dual-Band Mid-Wave and Long-Wave Infrared Reflectors

Abstract

We present an innovative method for dual-band mid-wave infrared (MWIR) and long-wave infrared (LWIR) reflectors. By using double-sided metasurfaces, two high reflection bands can be generated with a single device. As individual guided-mode resonance (GMR) reflectors are combined with interlayer (or substrate) on the top and bottom sides, we achieved high reflection in the MWIR and LWIR bands simultaneously. Each GMR reflector was optimized as a germanium (Ge) grating structure on a potassium bromide (KBr) substrate. In our analysis, it was found that the transparency of the interlayer is critical to produce the dual-band reflection. The simulation results on the Ge/KBr/Ge double-sided metasurfaces demonstrated wideband reflection from ~3.3 to 4.8 μm and ~8.8 to 11 μm. Additionally, the device exhibited favorable angular tolerance. The work contributes to developing capability of metasurface technologies in various application fields.

1. Introduction

Mid-wave infrared (MWIR, λ ≈ 3 to 5 µm) and long-wave infrared (LWIR, λ ≈ 8 to 14 µm) technologies play important common roles in thermal imaging, spectroscopy and remote sensing [1,2]. These wavelengths correspond to transparent atmospheric windows allowing high transmission of thermal radiation. MWIR imaging systems are capable of excellent resolution with enhanced sensitivity and contrast to temperature difference [3], while LWIR systems offer high quality imaging of near-ambient temperature objects and complex environmental conditions [4]. As a synergistic approach to merging these technologies, recent efforts have focused on combining MWIR and LWIR capabilities to yield enhanced performance and broader spectral coverage [5,6].

Meanwhile, optical metasurface technologies have been rapidly advanced through the implementation of resonant photonic lattices (PLs) [7,8]. Contrasting traditional thin-film technology, the PLs enable a single-layer operation for various optical components including band-pass filters [9], wideband reflectors [10] and polarizers [11]. Particularly, for the MWIR and LWIR bands, guided-mode resonance (GMR)-fashioned PLs are alternatives to traditional multi-layer designs based on quarter-wavelength thickness [12]. However, designing optical components that simultaneously operate effectively in both the MWIR and LWIR bands remains a considerable challenging in metasurface technology. In this work, we introduce an approach for dual-band operation via double-sided metasurface structures. As an essential optical component, we focus on design and analysis of MWIR and LWIR reflectors, potentially useful for dual-band operation in various application fields.

2. Double-Side Metasurface Reflector

As shown in Figure 1, double-sided metasurfaces enable high reflection in MWIR and LWIR bands. The dual-band reflection is attributed to cooperative interaction between two distinct resonant reflectors, optimally designed for each MWIR (3–5 μm) and LWIR (8–12 μm) region. When light illuminates the device, the first metasurface reflector generates a high reflection band in the MWIR spectral region while partially reflecting light in LWIR region, as depicted in (i). In an ideal lossless system, the remaining light entirely passes through the substrate and interacts with the second reflector, as shown in (ii). Thereafter, the LWIR light is mostly reflected into air. As a result, seen in (iii), light undergoes high reflection in both MWIR and LWIR regions. Regarding infrared transparent materials such as germanium (Ge), calcium difluoride (CaF₂), potassium bromide (KBr), here, we successfully develop and analyze double-sided metasurfaces for dual-band MWIR and LWIR reflectors.

3. Numerical Modeling

Dual-band reflectors as proposed here can be constructed using two individually optimized resonant wideband reflectors. In this work, we employ particle swarm optimization (PSO) algorithms [13] to achieve a high reflection in each 3–5 μm MWIR and 8–12 μm LWIR band. For calculation of the reflection, a rigorous coupled-wave analysis (RCWA) method [14] is used with a commercial software package (version 2023.12-SP1) [15]. As a simple construction, we treat a one-dimensional (1D) grating array based on high refractive index dielectric material where we keep 19 harmonics for reliable simulations.

Figure 2a presents the schematic of the 1D MWIR grating reflector. In this design, we incorporate a homogeneous sublayer beneath the one-dimensional (1D) grating using the same high refractive index material. This forms a zero-contrast grating (ZCG), which facilitates superior wideband reflection to achieve [16]. The grating parameter set {${\Lambda }_1,F_1,d_{g1},d_{h1}$} determines the geometry of the 1D ZCG, where the ${\Lambda }_1$, $F_1$, $d_{g1}$ and $d_{h1}$ denote a period, fill factor, grating depth, and sublayer thickness of the first part of the metasurface. By setting the refractive indices (n_h = 4, n_s = 1.4) and using TM polarization (i.e., electric field is perpendicular to the grooves), we determine the optimal parameter set as {${\Lambda }_1$ = 1.73 μm, $F_1$ = 0.73, $d_{g1}$ = 0.91 μm, $d_{h1}$ = 0.5 μm} for the MWIR reflector. The simulated zeroth-order reflectance (R₀) spectrum demonstrates a high reflection band spanning from 3.06 to 4.85 μm, with R₀ values exceeding 0.97 remarkably achieved by the single grating layer. The wideband reflection is attributed to the merging of multiple guided-mode resonance (GMR) modes in the subwavelength regime [17]. At longer wavelengths, high reflection does not appear because GMRs are not excited.

For supporting high reflection in the LWIR region, as shown in Figure 2b, the second metasurface is designed by increasing the grating period where the optimal parameter set is {${\Lambda }_2$ = 6 μm, $F_2$ = 0.38, $d_{g2}$ = 1.7 μm, $d_{h2}$ = 0.33 μm} under TE polarization (i.e., electric field is parallel to the grooves). In the simulated R₀ spectrum, wideband reflection is observed from 8.54 to 12.22 μm, with high reflectance (R₀ > 0.99). As the second metasurface, it can efficiently reflect LWIR light transmitted from the first metasurface.

Figure 2c shows a double-sided metasurfaces composed the of two individual reflectors from Figure 2a,b. The substrate thickness (d) is set to 1 mm, which is a commonly used value. To ensure that each reflector operates with the proper polarization, two 1D grating are aligned in orthogonal directions under linearly polarized light illumination. When the incident electric field is perpendicular to the first grating grooves, the first and second metasurface interact with TM and TE polarized electromagnetic waves, respectively. The simulation result of the R₀ spectrum clearly shows dual MWIR and LWIR reflection with high fluctuations due to Fabry-Pérot (FP) resonances.

4. Analysis

To explain a mechanism of the dual-band reflector, we provide an analytical formula based on a simplified optical configuration as illustrated in Figure 3a. The total output R₀ is derived from the sum of the nth reflectance ($R_{\left(n\right)}$) between the two metasurface which can be expressed by (1) to (3).

$$R_0=\sum _{n=0}^{\mathrm{\infty }}{R}_{\left(n\right)}$$

(1)

$${=R}_1+{R_2\left(1-R_1\right)}^2{e}^{-2\alpha d}\sum _{n=0}^{\mathrm{\infty }}{\left(R_1{R}_2{e}^{-2\alpha d}\right)}^n$$

(2)

$$=R_1+{\displaystyle \frac{{R_2\left(1-R_1\right)}^2{e}^{-2\alpha d}}{1-R_1{R}_2{e}^{-2\alpha d}}}$$

(3)

The R₁ and R₂ represent the reflectance of the first and the second metasurfaces, and α denotes the absorption coefficient of the interlayer (i.e., $\alpha =4\mathsf{\pi }k⁄\mathsf{\lambda }$, k being the extinction coefficient of the complex refractive index). In the equation, a phase interference is ignored to avoid FP resonance effects.

Figure 3b shows calculation results obtained by solving (3) using the R₁ and R₂ reflectance from each R₀ spectrum of Figure 2a,b. Comparing Figure 2c, which is in case of a lossless interlayer (extinction coefficient k = 0, depicted by the blue bold line), the analytical results match to the simulation (RCWA) results, except for the dense oscillation. In realistic fabrication, where mathematically perfect parallelism between two interfaces is unlikely to be formed, FP interference effects will be diminished. Thus, the double-sided metasurfaces can cooperate the two individual reflectors without the rapid fluctuation seen in the RCWA result, producing a response similar to that predicted by the analytical model. However, when the interlayer is lossy material, the dual-band reflection capability is significantly degraded. As shown in red bold line, which indicates analytical result for k = 0.001, R₀ dramatically decreases in the LWIR region while matching the RCWA results (red thin line). This substantial reduction is caused by the high absorption of the thick interlayer (d = 1 mm) as evidenced by the absorption (A) spectrum (i.e., RCWA results, depicted by the green thin line). The LWIR light is substantially absorbed during the long propagation, multi-wavelength path in the interlayer, whereas the MWIR light is predominantly reflected not penetrating the interlayer. Unfortunately, for this reason, it is hard to operate the second reflector when utilizing lossy materials for the interlayer.

5. Design and Results

As investigated in the previous section, transparency of the interlayer is critical to achieve the dual-band reflection. Prior to design, as seen in Figure 4a, we examined the zeroth-order transmittance (T₀) of 1 mm-thickCaF₂ and KBr substrates as the interlayer. The red line represents a measured T₀ spectrum of the CaF₂ substrate obtained by using a Fourier transform IR (FTIR) spectrometer (Thermo Scientific Nicolet iN10, Thermo Fisher Scientific, Waltham, MA, USA). In the MWIR region, it exhibits a high T₀~0.92 indicating that the light remaining after external reflection (R~0.08 at surface and bottom) propagates without internal absorption. However, the T₀ rapidly decreases when the wavelength is longer than λ = 8 μm due to the increased internal loss. The blue line shows a simulated T₀ spectrum where we have adjusted the CaF₂ dispersion to closely match the measured values based on previous experimental results [18]. At long wavelengths λ > 8 μm, the FP resonance is diminished by a fact that the internal absorption prevents the light from making a round trip within the cavity. The T₀ spectrum of a KBr substrate (green line) is also simulated by accounting for material dispersion based on previous experiments [19]. In both MWIR and LWIR regions, KBr exhibits apparently high transparency without absorption.

Figure 4b shows the design result of double-sided metasurfaces based on Ge/CaF₂/Ge material system where the first and second optimal grating parameter sets are {${\Lambda }_1$ = 1.725 μm, $F_1$ = 0.732, $d_{g1}$ = 0.9 μm, $d_{h1}$ = 0.49 μm} and {${\Lambda }_2$ = 6 μm, $F_2$ = 0.38, $d_{g2}$ = 1.7 μm, $d_{h2}$ = 0.32 μm} for the individual reflectors. The dispersion of Ge is accounted for [20]. As anticipated, dual-band reflection is not available due to the internal absorption of the CaF₂ interlayer as noted quantitatively by the significantly high absorbance (A) spectrum displayed in blue. Only the first reflector effectively operates in the MWIR region. In contrast, the Ge/KBr/Ge double-sided metasurfaces achieves the dual-band reflection without the internal absorption as presented in Figure 4c. In each region, high reflection is observed from 3.3 to 4.77 μm and 8.85 to 11 μm where the first and second optimal grating parameter sets are {${\Lambda }_1$ = 1.73 μm, $F_1$ = 0.73, $d_{g1}$ = 0.92 μm, $d_{h1}$ = 0.5 μm} and {${\Lambda }_2$ = 5.39 μm, $F_2$ = 0.38, $d_{g2}$ = 1.75 μm, $d_{h2}$ = 0.34 μm}.

To investigate the angular tolerance of the double-sided metasurfaces, we examine the dual-band reflection while varying two different angles. Figure 5a illustrates angular variation with the plane of incidence (POI) perpendicular to the grating grooves of the first reflector. In this case, the MWIR reflector is subjected to classical mounting while the LWIR reflector experiences fully conical mounting. In general, the classic incidence varies the resonance more since it induces greater deviation of the diffracted waves than conic incidence [21]. Likewise, as shown in the $R_0(\mathsf{\lambda },\mathsf{\theta })$ color map, it is observed that the MWIR reflection band is split under classic mounting, but the LWIR reflection band remains robust under fully conical mounting. When the POI is parallel to the grooves, as seen in Figure 5b, the first reflector undergoes fully conical mounting, thereby enhancing angular tolerance in the MWIR band. However, reflectance is somewhat degraded in the LWIR band due to the classical mounting of the second reflector for this case.

6. Discussion

We present an analysis and a design of dual-band resonant reflectors operating in the infrared spectral region. We emphasize the fact that the dual-band functionality is realized with a pair of single-layer films appropriately patterned laterally. The reflector is shown to exhibit wideband performance with high efficiency engaging guided-mode resonance, also termed lattice resonance, effects as physical basis. Thereby, efficient and simultaneous reflection of MWIR and LWIR spectra is achieved as presented in detail herein. Challenges due to material loss in available substrate media are explained.

In these bands, there is a dearth of available lossless dielectric materials that limit the supply and functionality of corresponding components for broad spectral handling and manipulation. This contrasts with wide availability of materials in the visible and near-IR domains where component technology is dominated by classic thin-film solutions that are widely offered commercially. We note that thin-film versions of the dual-band reflector presented here will be challenging in fabrication due to substantial thickness of quarter-wave films particularly in the LWIR band. Often demanded specifications on size, weight, and power (SWaP) attributes are very favorable for resonance lattice MWIR/LWIR reflectors relative to the metallic reflectors often deployed in these bands.

Physical construction of the proposed device is challenging due to the necessity of protecting one surface during the pattering of the other. Nevertheless, fabrication is feasible with careful processing and procedures. Alternate implementation involves the serial arrangement of individual resonators on separate substrates; corresponding devices have not yet been designed. The basic concept of dual reflection band operation is extendable to other spectral regions spanning 400–6000 nm wherein the material selection limitation is not as extreme as in the LWIR band. For polarization independence, the GMR resonators may be patterned as 2D metasurfaces.

Few comparable results exist in the literature. A dual-band reflector operating at two precisely specified frequencies was designed and fabricated for 6G applications and operating in the mm-wave region using the principles of diffractive optics [22]. The reflector consists of a 2D array of unit cells containing metallic elements on a dielectric substrate over a metallic ground plane. For a fixed incident direction of plane waves at 220 GHz and 293.3 GHz, the 3rd and 4th diffraction orders propagate colinearly in reflection with ~80% efficiency excluding inherent metal losses [22]. In another work, for lower frequencies, a high-gain low-profile reflector dual-band antenna was reported [23]. The antenna exhibited reasonable gain in bands centered at 31 GHz and 158 GHz. Here, the innovation was a periodic subwavelength array patterned in 2D to achieve a useful performance [23] but the operational principle is totally different from the lattice resonance concept presented herein. Finally, a multi-band reflector using sequential planes of metamaterials was proposed, designed, and fabricated to operate at still lower frequencies [24]. The metasurfaces were composed of wood-pile photonic crystals, a classic element operating principally on Bragg effects and not via GMR. Reflectance data in the 0.6–2.2 GHz band were presented [24].

These very-long-wave microwave reflectors are lossy, bulky, and complex. In contrast, the optical dual-band reflector presented here is ultracompact and simple. We estimate that the substrate could be as thin as ~200 μm while remaining robust and manageable in optical systems. It would appear as a single-layer device to a user, excellent for lightweight systems in space and air.

In closing, we consider the effect of incident illumination deviating from a perfect TM polarization state. As shown in Figure 6a, the double-sided 1D reflector of Figure 4c maintains the dual band of high reflection up to 10° in polarization angle ($\varphi$). The inset indicates the $\varphi$ between the POI and grating vector of the first reflector. However, the dual band is degraded by further increases in $\varphi =$ 20°, 30°, and 90° shown in Figure 6b–d. Polarization independence at normal incidence is attained via 2D photonic lattices.

7. Conclusions

In summary, we introduced a dual-band reflector that incorporates double-sided metasurfaces. This innovative design pertains to the cooperation of two GMR reflectors, where two metasurface are combined with an interlayer positioned between them. The top and bottom metasurfaces can be independently designed to achieve the desired spectral response and both functionalities are integrated into a single device with a transparent interlayer. For the MWIR and LWIR reflection bands, two 1D ZCG structures were optimized by utilizing Ge grating layers on a KBr substrate. The Ge/KBr/Ge design demonstrates wideband reflection from 3.3 to 4.77 μm (MWIR region) and 8.85 to 11 μm (LWIR region). In addition, we provided an analytical formula, which will be useful to estimate output spectral response of double-sided metasurfaces in this device class.

We note that the work presented is completely original with no similar results previously published. The contribution is solid on account of potential applications in imaging, night vision systems, thermal systems, and environmental monitoring. A future challenge is to fabricate examples of this device class while establishing attendant processes and methodology.